Knowledge of the vertical deflection of the gravity vector has great utility in various inertial navigation and/or geodetic applications. Conventional procedures to determine the two-dimensional deflection of the gravity vector require the use of a theodolite for stellar observations and clear night-time weather conditions. Therefore, this procedure is sensitive to environmental conditions.
More recently, the vertical deflection of the gravity vector has been related to changes in orthometric and geometric heights over a baseline as taught by Heiskamen and Moritz in "Physical Geodesy", W. H. Freeman and Company, San Francisco, 1967. Changes in orthometric heights are normally determined by a standard spirit leveling survey. These heights are referenced, where possible, to mean sea level. The leveling survey determines the change in orthometric height above sea level of the land surface. Change in geometric height is the difference in height above an ellipsoid model of the earth at two locations. This is normally determined using satellite relative positioning procedures.
The drawbacks of this approach include the time required to effect the determination of both the orthometric and geometric height measurements since both are determined separately. Furthermore, benchmarks indicating baselines must be used to establish the points of reference for each separate measurement. Thus, the separate determinations of orthometric and geometric height differences are inevitable sources of error. Also, satellite surveying benchmarks are often blocked from receiving the RF satellite signals. For these benchmarks, nearby offset locations must be chosen which can receive the RF satellite signals. It thus becomes necessary to relate the offset locations to the original benchmark of interest.